I have 4l of Castrol R4 superbike oil, 10w/40 fully synthetic i bought but never used for my zx636, would it be ok to use on my nc23k or is it a bit too poncy and modern ?

It fits as far as grade goes, im just curious if fully synthetic is suitable or if i should get some semi-synthetic instead.

Same subject, any recommendations for oil filter brand to go for ?

Also, i have a set of plugs in good nick that came out of my mates vfr750, they only did 2k miles and are clean as a whistle, surprisingly. Im planning on doing a service shortly and was wondering if the 30-40 notes for Iridiums will make a great deal of difference over this standard set of ngk's.

I have heard of clutch slippage with fully synthetic oil, but I don't know if it's a problem with CBR400s.
I used Castrol Power1 GPS on my last change, which is semi-synth.

I used Motul 10W40 fully synthetic not any problem, engine works brilliant and Repsol 10W40 Synthetic also not any issue.

Fully Synth does affect some bikes clutches.
i dont use it as i cant afford to mess around with wasting/changing oil. i use semi 10/40W

Depends on how good your clutch is.But the only way to tell is by putting it in

I wouldn't want to change a clutch just for superdooper oil.

Why don't you see if a shop will swap it for semi synth if its still sealed you might be able to get them to throw in a filter too.

Rock Oil Gaurdian 10/40W worked well for me in my old NC23